Recently, increased recording density has been desired in the area of magnetic recording. For attaining a high recording density, magnetic heads used in a hard disc drive have been changing from a ferrite head, which is conventionally used, to a thin-film magnetic head whose magnetic head element is formed by integrating technique which has been representatively used in the production of semiconductors. FIG. 2 is a plain view of a thin-film magnetic head, in which element 1 an insulating substrate or a head slider substrate; element 9 is an upper magnetic core and element 6 is a coil whose ends are connected to coil leads 7. The coil 6 is contained in a coil insulating layer 8. FIG. 3 is a sectional view at an X-X' cutting line shown in FIG. 2, in which an insulating layer 10 is formed on the insulating substrate 1. The insulating layer 10 is generally made of Al.sub.2 O.sub.3 or SiO.sub.2. After forming a lower magnetic core 11 on the insulating layer 10, a non-magnetic gap layer 12 is formed from the same material as the insulating layer 10. Over the gap layer 12, coil layers 6 are formed and covered with an insulating layer 8 which conforms to the shape of the coil layers 6. The insulating layer 8 is generally made of a heat-cured positive-type photoresist. On the insulating layer 8, an upper magnetic core 9 is formed and then a protective film 13 is formed thereon.
The thin-film magnetic head, as mentioned above, is required to have higher accuracy and higher integrating density. In the system for magnetic recording, high recording density enhances the sensitivity of a magnetic head, or enhances its output, because the leakage of the magnetic field, i.e. the information from the recording medium, becomes smaller. Since the output of the magnetic head is in proportion to number of coil windings, it is desirable to narrower the distance between coil wires and the width of the coil wire to result in an increase is the number of windings. This is also important in view of integrating magnetic head elements, because it is liable to make a magnetic head itself smaller.
FIGS. 4 A to 4 E schematically show a conventional process for forming a coil element. In FIG. 4 A, a base electrode 104 for coil plating is formed on an insulating substrate 101 by a dry process, such as sputtering, and then a mask 105 for coil plating is formed thereon with a photoresist, using photolithography (FIG. 4 (b)). Coil layers 106 are formed by electroplating (FIG. 4 (c)), and the mask 105 for coil plating is removed with a removing solution, such as an organic solvent (FIG. 4 (d)). Next, a portion of the base electrode 104, on which the photoresist mask 105 had been present, is removed by etching to form a coil pattern (FIG. 4 (e)).
In order to make the coil element small, to increase number of coil windings and to elevate an aspect ratio (a ratio of coil layer thickness/coil wire width), the coil wire width has been made narrower in comparison with a conventional coil element. The narrowed coil wire width, however, make it difficult or time consuming to remove the base electrode 104 between the coil layers 106. This often adversely affects on reduction of coil thickness, etching of coil side surface and the like, because the etching solution simultaneously erodes the coil layers 106. This defect is overcome by making the coil base electrode thin, but adhesivity between the coil and the surface of the insulating substrate, and uniformity of coil layers are poor.